Method of coating a mica base with magnesium hydroxide



2,715,585 Patented Aug. 16, 1955 I /iETHOD (BF COATL' IG A R HCA BASE WITH MAGNESIUM HYDROXIDE Waiter Frederick Lawrence, Jr., Nutley, N. J., assiguor to Radio Corporation of America, a corporation of Delaware No Drawing. Application August 30, 1951, Serial No. 244,463

7 Claims. (Cl. 117-62) My present invention relates to a method of forming an insulating coating of magnesium hydroxide having improved adherence to a base, and more particularly to a. novel method of forming such insulating coating on electrical insulators whereby the coating is resistant to separation from the insulators when either in a dry or wet state.

One type of electrical insulator is employed in electron tubes. This insulator comprises a fiat disc or plate made of an insulating material such as mica. The disc or plate has a plurality of apertures for receiving end portions of electrodes for spacing and also in some instances for supporting the electrodes.

While such discs or plates are made of insulating material, their relatively smooth surfaces become coated during manufacture and subsequently with evaporated getter and cathode material which form continuous electrical paths between two or more of the electrodes. To prevent formation of such continuous electrical paths, it has been the practice to apply an insulating coating to the discs or plates. Such coating provides a relatively rough surface on the insulators. The evolved getter and cath ode material is incapable of forming a continuous layer on such rough surface. Therefore, the rough coating prevents or at least substantially reduces electrical leakage between the electrodes.

A coating material heretofore used for forming insulating coatings on insulating spacer plates used in electron tubes is magnesium hydroxide.

One ditficulty experienced with magnesium hydroxide coatings is their inability to adhere Well both when dry and wet. Good adherence under both of these conditions is desirable. For example, good adherence when dry is desirable to prevent loss of coating during the handling of an electron tube sub-assembly that includes a coated spacer plate. Such handling is necessary in processing the sub-assembly for incorporation in an electron tube. Good adherence of the coating on the spacer plate when Wet is also advantageous. This is because sub-assemblies, such as mounts including a coated spacer plate, are sometimes washed in water prior to mounting in an electron tube.

It is found that when a coating of magnesium hydroxide is applied my-known methods, it is characterized by good dry adherence. However, the wet adherence of the coating is poor, so that it washes off during washing of the sub-assembly of which the coated insulating plate is a part. To secure good wet adherence of the coating, it has been the practice heretofore to fire sprayed insulating plates for five minutes at a temperature of 700 C. However, while this treatment improves the wet adherence of the coating, it causes the coating to adhere pooriy when dry. Therefore, the practices of the prior art were sub ject to the limitation that they could provide good adherence of an insulating coating only for one condition, i. e. when wet or when dry, but not for both of these conditions.

Accordingly, it is an object of the invention to provide 2 an insulating coating of magnesium hydroxide having improved adherence.

Another object is to provide a method for forming an insulating coating of magnesium hydroxide that is characterized by good adherence after application, under both dry and wet conditions.

A further object is to provide an improved method of applying and forming an insulating coating of magnesium hydroxide on an insulating spacer plate for an electron tube, that will have good adherence when dry to prevent loss of the coating during handling of the spacer plate prior to inclusion in the tube, and that will also have good adherence during a washing of a subassembly including the coated spacer plate.

Another object is to provide a method for forming an insulating coating of magnesium hydroxide on an insulating spacer plate whereby adherence of the coating on the plate is improved and coherence of the coating material is strengthened to thereby prevent loss of the coating during handling and during wet Washing of the coated plate.

Further objects and advantages of the invention will manifest themselves as the present description continues.

Briefly considered, the method of the invention includes the step of first spraying or otherwise applying to an article to be coated, such as an insulating spacer plate made of mica employed in electron tubes, a coating material comprising a suspension of magnesium hydroxide Mg(OH) The spraying step may be carried out in accordance with conventional practice.

The second step comprises firing the coating applied to the insulator during the first step. According to the invention, the firing step is carried out at a temperature of at least 325 C. and is continued for a sufficient time to permit the insulating spacer plate and the coating thereon to heat up.

The third step comprises wetting the fired coating with water. The fired coating is relatively porous so that it becomes saturated with water in a relatively short time.

Thereafter the coating is dried at a temperature below 324 C., such as room temperature.

The firing step referred to imparts good adherence of the coating to the spacer plate when wet. The subsequent wetting step improves the coherence of the coating material so that it possesses good resistance to loss during handling when dry. The method of the invention therefore provides an insulating coating of magnesium hydroxide that adheres well to the article coated in both wet and dry states.

A more detailed consideration of the invention will be helpful for a better understanding thereof.

The invention is concerned primarily with a method of forming a coating of magnesium hydroxide on an insulating spacer plate, that will remain on the spacer plate i during dry handling of an electron tube sub-assembly in which the plate is used and during washing of the subassembly in water. I have found that two factors are involved in assuring freedom from loss of the coating under the two conditions mentioned.

One of these factors involves cohesion of the particles of which the coating is made, or the bond that the particles exhibit with respect to each other. If this cohesion or bond is strong, the coating is free from loss during handling of the coated article. However, this characteristic of the coating of freedom from loss during handling,

is manifested only when the coating is dry. When the coating is wet, from washing the coated article in water, for example, another factor is involved.

This other factor concerns the adhesion of the coating on a base, such as an insulating spacer plate. The adhesion referred to is the bond between the base and the coating.

Modern tube manufacturing procedures require in some instances that tube sub-assemblies, such as mounts, which usually include one or more spacer plates sprayed with magnesium hydroxide, be washed in water to remove any foreign. matter that may have been picked up by a mount during handling, and which must be removed prior to assembly of the mount in an electron tube. To fix the coating to the spacer, plate so as to prevent loss thereof during the washing operation, it has heretofore been the practice to fire the coated plates at a temperature of about 700 C. It was found that this improves the adhesion or bond between the coating and base so that very little loss of coating occurred during the washing. But unfortunately, this relatively high firing temperature had the undesirable eifect of reducing the coherence of the coating particles, so that appreciable loss of coating occurred during handling of a coated article.

It was impractical to delay the firing step until procedures requiring handling of the coated spacer plates had been completed. This was because the procedures referred to culminated in a finished sub-assembly foran electron tube generally called a mount. To heat the mount to the relatively high temperature required in firing the coating might 'cause damage to other components of the mount, such as a cathode, for example. This limitation continues to characterize electron tube manufacture, so that it is necessary to complete the processing incidental to the application of the insulating coating to the spacer plates, prior to inclusion of the plates in a mount.

According to the invention, a method is provided for coating an insulating spacer plate With magnesium hydroxide, prior to inclusion of the plate in an electron tube. 'The coating applied by the method has good adherence to the plate so that'it'is free from loss during the washing of a mount, including the plate, and it is also characterized by good cohesion so that it does not come oif during handling of the mount when dry.

The first step of the coating method of the invention is to spray or otherwise apply a coating of magnesium hydroxide to an insulating spacer plate. 'The initial application of the coating may be effected in accordance with conventional practices.

The spray material is in theform of a suspension of magnesium hydroxide in water. While other forms of magnesium hydroxide may be used in practicing the method of the invention, it is preferred to use a form thereof known commercially as Hydro- Magma. This form is preferred because 'of its relatively small and uniform particle size. The small and uniform particle size referred to results in a coating having a uniform thickness. The next step in practicing the method of the invention is to fire the coating applied during the first step. The firing step must be carried out at a temperature no lower than 325" (3., although ahigher temperature is permissible. Temperatures of 325 C. to 700 C. have been tried and found to give good results. While there is no'reason to' limit the temperature to an upper limit of 700 C. insofar as desirable results are concerned,

it is not necessary to go to a higher temperature than 325 C. Temperatures higher than this have the ob jection that they involve unnecessary energy consumption. It is preferable, however, in accordance with the invention, to employ a temperature higher than 325 C. to

'make certain that the firing temperature is at least 325 C.

The temperature of 325 C. referred to is critical as the lower limit of the firing temperature range of the invention, because this is the'reaction temperature of magnesium hydroxide and because I have found it is necessary to heat the coated article to at least the reaction temperature of the coating. The reaction occurring at this temperature is Mg. (OH)2 plus heat of at least 325 C.=Mg 0 plus H2O.

I believe this reaction has a markedeffect on the adherence of the coating on the base. While I do not wish to be bound by any particular theory in explanation of V the improved adherence of the coating, I believe it oche chemical in nature,

curs as a consequence of the strong tendency of the nascent or newly formed magnesium oxide to react with adjacent materials.

jacent the base or spacer plate is therefore believed to react with the base to form a strong bond therewith. This reaction may be a step referred to.

The reaction referred to takes place almost instantaneously. The time duration of the firingstep is therefore only required to be long enough to magnesium hydroxide, causes the formation of mag-. nesium oxide, I believe a coating including magnesium' oxide is characterized by a relatively poor cohesion, so

that it has a tendency'to come off during handling, by sticking to the hands of an operator or other agencies brought into contact with the coating. By subjecting the coating to the wetting step' referred to, there follows'a reconversion of themagneslum oxide to magnesium hydroxide, which after treatment according to the method of the invention has greater resistance to loss during handling whether wet or dry.

The wetting'step may be carried out in difierent ways, 7

for example by dipping the coated article in a water reservoir, spraying the article with water, or otherwise.

The Wetting step requires no particular control as to the time involved. All that isnecessary is that. the coating be wetted thoroughly. Due to the porous character of the magnesium oxide, it absorbs water relatively fast, so that a period of less than one minute for carrying out the wetting step has been found adequate. Excessive wetting does no harm in view of the following and final step in the method of the invention.

This final step involves drying the wetted coatedarticle. The drying step is carried out at a relatively low temperature such as room temperature, for example. Such low temperature should be below the reaction temperature of magnesium hydroxide. In view of this limitation on the drying temperature, it may theoretically ave a value of 324 C. However, as a precaution, it is preferred to use an appreciably lower temperature during the drying step. At room temperature no possibility of the reaction referred to exists, and this temperature is therefore preferred. But if more rapid drying isdesired, a higher temperature up 10324" C. maybe used.

It is apparent from the foregoing that a novel and advantageous method is provided for coating an electrical insulator with an insulating coating of magnesium hydroxide. By the step of firing the coated article, the coating is given the desirable characteristics of good adherence to the article which is effective both when the article is wet as well as dry. As a consequence of Wetting the fired coating, the coating is rendered more coherent, both when Wet as well as when dry, so that it is characterized by greater freedom fromlcss during-handling.

The method of the invention thus contributes appreci- The magnesium oxide formed admechanical interdifiusion of the newly formed magnesium oxide into the base, or it may or both. 'But whatever may be the reason, I have found'that the bond between the coating and base is considerably strengthened by the firing V of magnesium hydroxide on said insulator to provide a coating thereon, firing said coating at a temperature above 325 C. and until the coated insulator reaches said temperature at least instantaneously, to transform at least a portion of said coating to magnesium oxide, water wetting until substantially saturated said fired coating to retransform at least a portion of said magnesium oxide to magnesium hydroxide, and drying said wetted coating at a moderate temperature below 325 C., whereby said coating is resistant to separation from said insulator during dry handling and washing in water.

2. Method of coating :1 mica insulator with magnesium hydroxide comprising the steps of applying to a surface of said insulator a coating of an aqueous dispersion of magnesium hydroxide, firing said coating for five minutes at a temperature of from 325 C. to 700 C. to convert substantially all the magnesium hydroxide of said coating to magnesium oxide for fixing said coating to said insulator, subsequently water wetting said coating to saturation to reconvert substantially all of said magnesium oxide to magnesium hydroxide for improving the coherence of said coating, and drying the water wetted coating at a temperature below 325 C.

3. Method of coating a mica article with magnesium hydroxide particles to provide a coating having improved adherence on said article and characterized by relatively strong coherence of said particles, said method comprising the steps of applying to a surface of said article a coating of an aqueous dispersion of magnesium hydroxide, heating said coating to cause said coating momentarily to have a temperature between 325 C. and 700 C., to form desirable nascent magnesium oxide in said coating adjacent said article for establishing a relatively strong bond between said coating and article, and to cause magnesium hydroxide in said coating in regions thereof relatively remote from said article to react at said temperature to form magnesium oxide, wetting said coating with Water for about one minute to cause it to become water saturated for reconverting substantially all of said magnesium oxide, to magnesium hydroxide for providing improved coherence of the particles of said coating, and drying said coating at room temperature.

4. In a method of coating a mica insulator including the steps of applying a coating of water wetted magnesium hydroxide to said insulator and firing the coating for five minutes at a temperature above 325 C., the improvement comprising the steps of dipping the coated insulator in water after said firing step to water saturate said coating, and drying said coating at room temperature, whereby said insulator is adapted to be washed and to be handled when dry with reduced loss of said coating.

5. Method of coating a mica insulator with magnesium hydroxide to provide a coating having both good adherence to said insulator and good coherence for reducing coating loss both during washing of the coated insulator and handling thereof when dry, said method compris ing the steps of applying an aqueous dispersion of magnesium hydroxide to said insulator to coat a surface thereof, firing the coated surface at a temperature from 325 C. to 700 C. for five minutes to convert at least a portion of said magnesium hydroxide to magnesium oxide, rewetting with water the fired coated surface to substantially saturate the coating on said surface, and drying the rewetted coated surface, at a temperature below 324 C.

6. Method of coating a mica insulator with magnesium hydroxide to provide a coating having improved adherence to said insulator for reduced coating loss when washing the coated insulator, and having improved coherence for reducing coating loss when handling said coated insulator with said coating dry, said method comprising the steps of applying an aqueous suspension of magnesium hydroxide to said insulator to form a coating thereon, firing said coating at a temperature at least as high as 325 C. for five minutes to transform at least a portion of said coating to magnesium oxide, Water wotting the fired coating substantially to saturation to convert at least a portion of said magnesium oxide to magnesium hydroxide, and drying said coating at a temperature from room temperature to 324 C.

7. In a method of coating a mica base including the steps of spraying on said base water wetted magnesium hydroxide and firing the coated base at a temperature above 325 C. to convert at least a portion of the magnesium hydroxide adjacent said base to magnesium oxide, the improvement comprising the steps of wetting the fired coating with water substantially to water saturate said coating, and drying the wetted coating at temperature between room temperature and 324 C., whereby said coating is characterized by improved adherence and coherence for reduced loss of coating during dry handling and Washing of the coated base.

Elsey Aug. 4, 1942 Cardell Jan. 11, 1944 

1. METHOD OF COATING A MICA INSULATOR WITH MAGNESIUM HYDROXIDE TO REDUCE LEAKAGE PATHS ACROSS SAID INSULATOR, SAID METHOD COMPRISING SPRAYING AN AQUEOUS DISPERSION OF MAGNESIUM HYDROXIDE ON SAID INSULATOR TO PROVIDE A COATING THEREON, FIRING SAID COATING AT A TEMPERATURE ABOVE 325* C. AND UNTIL THE COATED INSULATOR REACHES SAID TEMPERATURE AT LEAST INSTANTANEOUSLY, TO TRANSFORM AT LEAST A PORTION OF SAID COATING TO MAGNESIUM IXODE, WATER WETTING UNTIL SUBSTANTIALLY SATURATED SAID FIRED COATING TO RETRANSFORM AT LEAST A PORTION OF SAID MAGNESIUM OXIDE TO MAGNESIUM HYDROXIDE, AND DRYING SAID WETTED COATING AT A MODERATE TEMPERATURE BELOW 325* C., WHEREBY SAID COATING IS RESISTANT TO SEPARATION FROM SAID INSULATOR DURING DRY HANDLING AND WASHING IN WATER. 